Continuous-flow dryers are dryers, in which material to be dried is transported continuously or in batches through the dryer. Such a dryer is in particular a belt dryer which conveys the material to be dried through the continuous-flow dryer by means of a belt. The material to be dried, for example sewage sludge, wood chips, hogged wood, RDF (refuse-derived fuel), SSW (solid shredded waste), MSW (municipal solid waste), household waste, grass or agricultural products and by-products such as sugar beet pulp, is here at first damp or wet. The material is dried by moisture being removed from it by means of hot air. The hot air is separately produced by heating in particular air from the surroundings of the continuous-flow dryer. On heating the air, the relative air moisture of this air decreases, the air becomes “drier”. This hot air with low relative air moisture then flows through and around the constituents to be dried in the continuous-flow dryer.
For heating the air to hot air, energy is of course needed. This energy is lost when the hot air generated is discharged into the surroundings after the drying of the material. First steps for circulating the hot air are therefore known.
The material to be dried is at the same time conveyed in a transport direction through the continuous-flow dryer and in so doing preferably runs through a plurality of sections. The individual section divides the continuous-flow dryer spatially. The sections can for this purpose be largely separated from one another in terms of air flow. Different air flows are thus possible in the sections, which can each have different relative air moistures and different temperatures.
For supplying air to the continuous-flow dryer a fresh air supply device is provided, which supplies generally dry fresh air, removed from the surroundings, to the continuous-flow dryer as supply air.
In the case of such continuous-flow dryers it is further known to provide an exhaust air recirculation device, by means of which exhaust air is removed from the drying process and then at least partly recirculated into the continuous-flow dryer. In so doing, part of the exhaust air is led through a heat exchanger, through which also supplied fresh air is led. Thus, thermal energy or waste heat from the exhaust air can be transferred to the supply air.